Methods and systems for characterizing lcm particle plugging and rheology in real time

ABSTRACT

Methods and systems for characterizing drilling fluids laden with LCM (Lost Circulation Material) and other solid materials are disclosed. A test cell for analyzing a fluid is provided with a first conical inner portion and an axial positioning device positioned along an axis of the test cell. A first conical plug is coupled to the axial positioning device and is movable in and out of the first conical inner portion along the axis of the test cell. A fluid inlet is positioned at a first location on the test cell and a fluid outlet at a second location.

This application is a divisional of U.S. application Ser. No. 12/328,836filed on Dec. 5, 2008.

BACKGROUND

The present invention relates generally to methods and systems formaterial characterization and more particularly, to methods and systemsfor characterizing drilling fluids laden with LCM (Lost CirculationMaterial) and other solid materials.

Drilling operations play an important role when developing oil, gas orwater wells or when mining for minerals and the like. During thedrilling operations a drill bit passes through various layers of earthstrata as it descends to a desired depth. Drilling fluids are commonlyemployed during the drilling operations and perform several importantfunctions including, but not limited to, removing the cuttings from thewell to the surface, controlling formation pressures, sealing permeableformations, minimizing formation damage, and cooling and lubricating thedrill bit.

When the drill bit passes through porous, fractured or vugular stratasuch as sand, gravel, shale, limestone and the like, the hydrostaticpressure caused by the vertical column of the drilling fluid exceeds theability of the surrounding earth formation to support this pressure.Consequently, some drilling fluid is lost to the formation and fails toreturn to the surface. This loss may be any fraction up to a completeloss of the total circulating drilling fluid volume. This condition isgenerally known in the art as Lost Circulation. Failure to control LostCirculation increases drilling cost and can damage formation productioncapabilities.

The general practice is to add any number of materials to the drillingfluid which act to reduce or prevent the outward flow of the drillingfluid in a porous and or fractured stratum thereby reducing orpreventing Lost Circulation. The materials used in this process arecommonly referred to as Lost Circulation Materials (“LCM”). Somematerials typically used as LCM include, but are not limited to, woodfiber, popped popcorn, straw, bark chips, ground cork, mica, ground andsized minerals and the like.

In order to better understand the performance of a drilling fluid ladenwith LCM and/or other solid materials on the field, it would bedesirable to characterize and study the drilling fluid. Currently, suchtests are performed in the field. Field test are currently centered onthe standard API configured HTHP filtration device. In this device theuser can select porous media of various pore throat sizes. In someinstances a flat plate with a slotted gap(s) has been used. However,performing such tests on the field has several disadvantages.

One disadvantage of the current approach is that the drilling fluidcannot be analyzed in detail since the analysis will be limited to theexisting equipment such as the existing slot widths and angles.Moreover, performing such analysis in the field would be expensive andtime consuming.

FIGURES

Some specific example embodiments of the disclosure may be understood byreferring, in part, to the following description and the accompanyingdrawings.

FIG. 1 is a characterization system in accordance with an exemplaryembodiment of the present invention.

FIGS. 2A-2F depict the steps in preparing a sample drilling fluid.

FIG. 3 is a test cell in accordance with an exemplary embodiment of thepresent invention.

FIG. 4 is an enlarged view of a plug formed in a test cell in accordancewith an exemplary embodiment of the present invention.

FIG. 5 depicts a graphical representation of the simulation of someexpected test data from a test cell in accordance with an exemplaryembodiment of the present invention.

FIG. 6 depicts a test cell in accordance with an exemplary embodiment ofthe present invention.

FIG. 7 depicts a test cell in accordance with an exemplary embodiment ofthe present invention.

FIG. 8 depicts a test cell in accordance with an exemplary embodiment ofthe present invention.

While embodiments of this disclosure have been depicted and describedand are defined by reference to example embodiments of the disclosure,such references do not imply a limitation on the disclosure, and no suchlimitation is to be inferred. The subject matter disclosed is capable ofconsiderable modification, alteration, and equivalents in form andfunction, as will occur to those skilled in the pertinent art and havingthe benefit of this disclosure. The depicted and described embodimentsof this disclosure are examples only, and not exhaustive of the scope ofthe disclosure.

SUMMARY

The present invention relates generally to methods and systems formaterial characterization and more particularly, to methods and systemsfor characterizing drilling fluids laden with LCM (Lost CirculationMaterial) and other solid materials.

In one embodiment, the present invention is directed to acharacterization system comprising: a pilot testing mixer system; a LCMstripping system coupled to the pilot mixer system; and a test cellcoupled to the LCM stripping system.

In another exemplary embodiment, the present invention is directed to atest cell for analyzing a fluid comprising: a first conical innerportion; an axial positioning device positioned along an axis of thetest cell; a first conical plug coupled to the axial positioning device;wherein the first conical plug is movable in and out of the firstconical inner portion along the axis of the test cell; a fluid inlet ata first location on the test cell; and a fluid outlet at a secondlocation on the test cell.

In another exemplary embodiment, the present invention is directed to amethod of measuring the rheology of a first fluid comprising: passingthe first fluid through a gap formed between a conical plug and aconical portion of a test cell; measuring a pressure drop along the gap;using the pressure drop measurement to determine a shear stress;measuring the flow rate of the first fluid through the gap; using theflow rate measurement and flow geometry to determine an average shearrate; and predicting rheological model parameters of the first fluidusing the shear stress and the average shear rate.

In another exemplary embodiment, the present invention is directed to amethod of optimizing sealing efficiency comprising: creating a gapbetween a conical plug and a conical portion in a test cell; wherein thegap width simulates a fracture width; flowing a first fluid through thegap; determining the sealing efficiency of the first fluid; clearing thegap; flowing a second fluid through the gap; determining the sealingefficiency of the second fluid; and determining which of the first fluidand the second fluid is more effective in sealing the gap. In oneexemplary embodiment, the present invention is directed to a method ofoptimizing sealing efficiency comprising: creating a gap between aconical plug and a conical portion in a test cell; wherein the gap widthsimulates a fracture width; flowing a first fluid through the gap;determining the sealing efficiency of the first fluid; flowing a secondfluid through the gap; determining the sealing efficiency of a mixtureof the first fluid and the second fluid; and determining if the secondfluid enhanced the sealing efficiency of the first fluid.

In another exemplary embodiment, the present invention is directed to amethod of determining an optimal performance range for a fluidcomprising: creating a gap between a conical plug and a conical portionin a test cell; flowing a fluid through the gap; determining the sealingefficiency of the fluid while changing the gap width; identifying arange of optimal performance gap widths for the drilling fluid.

The features and advantages of the present disclosure will be readilyapparent to those skilled in the art upon a reading of the descriptionof exemplary embodiments, which follows.

DESCRIPTION

The present invention relates generally to methods and systems formaterial characterization and more particularly, to methods and systemsfor characterizing drilling fluids laden with LCM (Lost CirculationMaterial) and other solid materials.

FIG. 1 depicts a characterization system 100 in accordance with anembodiment of the present invention. In one exemplary embodiment, thecharacterization system 100 comprises a Pilot Testing Mixer (PTM) system102, a LCM Stripping system 104 coupled to the PTM system 102 and a testcell 106 coupled to the LCM Stripping system 104. As would beappreciated by those of ordinary skill in the art, two components aredeemed coupled to each other when fluid can flow from one to the other.Moreover, coupling does not require that the components be directlyconnected.

FIGS. 2A-2F depict the operation of the PTM system 102 where thedrilling fluid to be analyzed is prepared. The PTM system 102 providesfor addition and mixture of known quantities of LCM products to thedrilling fluid. The PTM system 102 comprises a mixing tub 202 where thedrilling fluid mixture is prepared. As depicted in FIG. 2A, a mud supplypump 204 first adds the drilling mud to the mixing tub 202. Once thedrilling mud is added to the mixing tub 202 (FIG. 2B), the mixing tub202 is placed in position for addition of different products from theproduct storage units 206, 208, 210. Product 1 (206), Product 2 (208)and Product 3 (210) are added to the mixing tub 202 as depicted in FIGS.2C, 2D and 2E respectively. Although three products are depicted asbeing added to the mixing tub 202, as would be appreciated by those ofordinary skill in the art, with the benefit of this disclosure, one ormore products may be added to the drilling mud depending on the drillingfluid being analyzed. Moreover, in one embodiment, there may be noproducts added to the drilling mud in order to analyze the drilling muditself. Products 1, 2, and 3 may be LCM or other materials suitable foraddition to the drilling mud.

Once all the products are added to the mixing tub 202, a closure device212 closes the mixing tub 202 and mixes the contents therein preparing adesired drilling fluid mixture in the mixing tub 202. Each of the mudsupply pump 204 and the product storage units (206, 208, 210) are ineffect removably couplable to the mixing tub 202 and can be coupled tothe mixing tub 202 for addition of materials and then be removed.Similarly, the closure device 212 is removably connectable to the mixingtub 202 and can be removed therefrom once it has performed the mixingoperation. A pump 112 may then be used to deliver the drilling fluidmixture from the mixing tub 202 to the test cell 106. In one embodiment,a positive displacement pump may be utilized to deliver the drillingfluid mixture to the test cell 106.

FIG. 3 depicts an enlarged view of the test cell 106 which is where themeasurements are actually made. The test cell 106 comprises a conicalplug 302 coupled to an axial positioning device 304. The axialpositioning device 304 may be used to axially position the conical plug302 in a conical portion 306 formed by the test cell walls 308. The gapbetween the conical plug 302 and the test cell wall 308 simulates afracture width. Different fracture widths may be simulated by moving theconical plug 302 in and out of the conical portion 306 in the directionindicated by the arrow 310. In addition to simulating the fracturewidth, the geometry of the test cell wall 308 can be specified so as tosimulate a particular desired fracture angle. Consequently, the conicalplug 302 and the test cell wall 308 can be utilized to simulate a rangeof fracture widths and angles providing the capability of analyzing thedrilling fluid using a variable width slot. Stated otherwise, the slotwidth variability allows one to characterize the plugging and bridgingof LCM products through a variety of user selectable fracture widths.The test cell also comprises a fluid inlet 312 and a fluid outlet 314.In one embodiment a brush 316 or other cleaning device may be coupled tothe axial positioning device 304. The cleaning device may comprise oneor more brushes or sponges. Alternatively, ultrasonic cleaning devicesor jets may be used for cleaning the test cell walls 308. Anothercleaning device could be positioned so that it cleans the plug 302. Thebrush 316 may be used to clean the test cell 106. After each testsequence the axial positioning device 304 may be used to move the brush316 and perform a series of brush and rinse cycles to clean the testcell 106. As would be appreciated by those of ordinary skill in the art,with the benefit of this disclosure, a number of different materials maybe used during the rinse cycle depending on the drilling fluid beingtested. In one embodiment base oil or water may be used to rinse thetest cell 106. The materials removed from the test cell 106 aretransferred to a waste container 118.

Returning now to FIG. 1, in one embodiment the test fluid is pumpedthrough the test cell 106 at a constant rate while measuring thedifferential pressure across the simulated fracture at 108. As would beappreciated by those of ordinary skill in the art, with the benefit ofthis disclosure, the differential pressure may be measured in a numberof ways, including, but not limited to pressure transducers which may beused in pairs or differential pressure transducers. In another exemplaryembodiment, the drilling fluid may be analyzed by setting thedifferential pressure and controlling the flow rate until pluggingoccurs. In one embodiment, a positive displacement pump 110 may beutilized to control the flow of the drilling fluid through the test cell106. Although FIG. 1 depicts a positive displacement pump 110 configuredas a syringe pump, as would be appreciated by those of ordinary skill inthe art, with the benefit of this disclosure, any positive displacementpump capable of operating at the desired test pressure range may beused.

FIG. 4 depicts a simulated particle plugging and bridging in a test cell106 in accordance with an embodiment of the present invention. As thedrilling fluid is passed through the test cell 106 the LCM solids thatbridge and plug 402 the simulated fracture in the conical portion 306between the conical plug 302 and the test cell wall 308 may form a sealanywhere along the flow path. In the parallel slot mode, where the wallsdefining the slot are substantially parallel to one another, mostplugging is likely to occur at or very near the entrance of thesimulated fracture. In contrast, in the conical slot mode, where thewalls defining the slot form a tapered slot, the plugging initiationlocation could be anywhere along the conical test cell wall 308,depending on Particle Size Distribution. As would be appreciated bythose of ordinary skill in the art, with the benefit of this disclosure,in the tapered slot mode ultrasonic methods may be used to determine thebridging and sealing efficiencies along the simulated fracture path ifdesired.

FIG. 5 depicts a graphical representation of the simulation of someexpected test data from the test cell 106. The graph 500 depicts threedifferent gap width scenarios labeled Gap Width 1 through Gap Width 3based on the change in differential pressure over time. The Gap Width 1curve represents a scenario where the fracture plugs quickly and fluidflow through the fracture is completely shut off. The Gap Width 2 curverepresents a scenario where the fracture plugs more slowly, but doeseventually plug. Finally, in the curve labeled Gap Width 3 the flowcontinues and the fracture does not plug. The graph suggests that thefluid was treated sufficiently to plug the Gap Width 1. Because GapWidth 2 did plug but required more volume, the concentration of theideal particle size was lower but yet at sufficient concentration topermit plugging. Therefore, the required particle availability could beestablished.

In one embodiment, the test cell 106 may be used to provide insitu andreal time testing of various product mixtures to optimize sealingefficiency. As would be appreciated by those of ordinary skill in theart, with the benefit of this disclosure, a mixture has a high sealingefficiency if it can seal a fracture quickly and/or with the leastamount of LCM materials. Stated otherwise, the simulated fracturestructure of the test cell may be utilized as a pilot testing mechanismto minimize fluid loss and optimize LCM product usage. The methods andsystems disclosed herein enable a determination of the sealingefficiency of a drilling fluid as a function of fracture width based onfactors including, but not limited to, the rate of sealing and the totalfluid loss for a fixed time period. The test cell 106 may first be usedin the optimization mode thereby determining the best solution for agiven fracture width. Specifically, the gap width in the test cell 106may be configured to simulate a particular fracture width. A firstdrilling fluid is then passed through the gap and the efficiency of thatdrilling fluid in sealing the gap is determined. Next, after clearingthe gap, a second drilling fluid is passed though the gap and itssealing efficiency is determined. The results are then compared todetermine which of the first or the second drilling fluid performed moreefficiently in plugging that gap. As would be appreciated by those ofordinary skill in the art, with the benefit of this disclosure, thesealing efficiency of a drilling fluid may be determined by determiningthe rate and ultimately the volume of the drilling fluid required topass through the geometry before sealing occurs, with a smaller volumeindicating a greater sealing efficiency. The same steps may be repeatedin order to compare the sealing efficiency of a number of differentdrilling fluids for a particular gap width. The solution may then betested at wider and narrower simulated fracture widths to determine arange of optimal performance for a particular solution, therebyminimizing performance uncertainty.

In an alternative embodiment, the gap is not cleared after the firstdrilling fluid is passed therethrough. Instead, after determining thesealing efficiency of the first drilling fluid, a second drilling fluidis passed through the gap. The sealing efficiency of the mixture of thefirst drilling fluid and the second drilling fluid is then measured todetermine if the addition of the second drilling fluid has enhanced thesealing efficiency of the first drilling fluid.

In another exemplary embodiment, the test cell 106 may be used tomeasure the rheology of the drilling fluid. As would be appreciated bythose of ordinary skill in the art, with the benefit of this disclosure,an invariant description of the flow properties of a rheologicallycomplex fluid requires measurements in a steady or viscometric flow.There are three classes of such viscometric flow which include: (1) flowthrough a circular tube (Poiseuille), (2) flow through a thin slot oraxially between concentric cylinders (Plane Poiseuille), and (3) flowbetween coaxially concentric rotating cylinders (Couette). In oneembodiment, the present invention is directed to evaluating the rheologyof an LCM fluid from measurements in the Plane Poiseuille class usingformulae known to those of ordinary skill in the art, for computingnominal shear rate and shear stress from the flow rate and the pressuregradient.

In this mode of operation the configuration of the conical plug 302, theconical portion 306 and the variable slot width created provide thefundamental components of a rheometer. In this embodiment, the test cell106 configuration may be modified as depicted in FIG. 6. The rheometerconfiguration test cell 600 may have a longer conical portion 606 andconical plug 602 and a different location for the differential pressuretransducer 610 may be desirable as depicted in FIG. 6. The shear stressof the system may be determined by measuring the pressure drop along theconical portion 606 which simulates an annulus. Additionally, the flowrate would effectively provide an average shear rate. The resultingshear rate and shear stress values may be used to generate data topredict the rheological model parameters of the flow geometry likePlastic Viscosity (PV), Yield Point for the Bingham model (YP) and thedifferent parameters of the Herschel-Bulkley model (n (power lowexponent), k (consistency), and tau0 (yield stress). The shear rate inthe conical portion 606 is not constant. However, as would beappreciated by those of ordinary skill in the art, with the benefit ofthis disclosure, it would be adequate to provide a simple method toprovide basic mud engineering PV and YP, since the basic measurementsfor these numbers are at the higher shear rates and may be lesssensitive to the geometry constraints. Moreover, as would be appreciatedby those of ordinary skill in the art, with the benefit of thisdisclosure, the taper angle may be configured to approximate a constantshear rate.

In one embodiment the characterization system 100 may be used to obtaintreated fluid rheology by comparing the pressure drops of a LCM ladenfluid to the LCM stripped fluid. In typical operations the conventionalrheometers cannot characterize the viscosity increase of a LCM treatedfluid because of these rheometers intolerance to certain particle sizes.Thus, the pressure drop comparison, or ratio, may be used to calculatean effective viscosity increase due to the LCM loading based on thefollowing mathematical assumption:

U*≡U_(f)/U₀≈δ_(f)/δ₀≈dp_(f)/dp₀

where U* is a non-dimensional viscosity ratio; U_(f) represents theviscosity of the treated fluid; U_(o) represents the viscosity of theuntreated fluid; δ_(f) represents the shear stress of the treated fluid;δ₀ represents the shear stress for the untreated fluid; dp_(f)represents the pressure drop of a treated fluid; and dp₀ represents thepressure drop of the untreated fluid as measured in the test cell. Inone embodiment the ratio of the treated fluid pressure drop to theuntreated fluid pressure drop may be used in conjunction withconventional shear rate, shear stress measurements of untreated fluid toapproximate the treated fluid rheology. Untreated fluid rheology istypically measured by a FANN viscometer, available from HalliburtonEnergy Services of Duncan, Okla. In this analysis, the shear stress ateach shear rate of the untreated fluid is simply multiplied by U* toobtain the treated fluid shear stress data at that shear rate. Thesedata then can then be processed into any suitable rheological modelparameters and used in hydraulics equations. Consequently, the systemdisclosed herein would provide the real time rheological data for atreated fluid necessary to provide hydraulic calculations for LCMtreated fluids that are all but impossible to measure in the field withconventional equipment.

Additionally, the test cell 106 disclosed herein provides the ability tomeasure the rheology of the LCM laden fluid relative to that of the LCMparticle free fluid. The ability to characterize the treated fluidrheology would enable one to do hydraulic calculations prior toutilization of a treated fluid. This would ensure that the higherviscosity treated fluids will not cause the Equivalent CirculatoryDensity excursions beyond the fracture gradient during treatmentapplications or normal drilling.

In one exemplary embodiment the characterization system 100 of thepresent invention may be placed on a rig site permitting drilling fluidanalysis prior to drilling through known trouble zones. Specifically,the characterization system 100 may systematically test a series ofproduct additions prior to fluid exposure in a known problem zone. Inone embodiment the test treatments may be selected in a number of ways,including, but not limited to using DFG Solids Modeling softwareavailable from Halliburton Energy Services of Duncan, Okla. Once testedand verified, the instrument will provide data to enable product andconcentration recommendations, thereby providing high quality real timesolutions to lost circulation problems. In another embodiment, themethods of the present invention may be employed during troublemitigation. In this embodiment, when a problem zone is anticipated,various treatment scenarios may be tested to ensure appropriatetreatment during the drilling process. As would be appreciated by thoseof ordinary skill in the art, with the benefit of this disclosure, someprior knowledge of what to expect typically comes from offset well data,

In yet another exemplary embodiment the characterization system 100 maybe used to verify whether the current LCM loading is adequate. In thismode of operation the test cell 106 would test the drilling fluid in anas received condition. Once the drilling fluid is tested by the testcell 106 it is passed through the LCM stripping system 104. The LCMmaterial is then filtered out by the filter 114 and transferred to awaste container 116. The base mud exiting the filter may then be passedback to the test cell 106 to be analyzed. In this mode of operation thecharacterization system 100 may be utilized to quantify the pluggingefficiency of the current LCM treatment as compared to the base mud. Inanother embodiment, once the base mud exits the filter 114 it isforwarded to the pilot testing mixing system 102. The pilot testingmixing system 102 may then introduce new LCM material(s) into thedrilling mud which may be then passed back to the test cell 106 tocompare the characteristics of different LCM treatments. As would beappreciated by those of ordinary skill in the art, with the benefit ofthis disclosure, the LCM stripping system 102 may be cleaned in a numberof ways. In one embodiment, the LCM stripping system 102 may be cleanedby back flushing with a clean base fluid.

Depicted in FIG. 7 is a test cell in accordance with another exemplaryembodiment of the present invention. In this embodiment, the test cellcomprises two conical plugs 702, 704 coupled to an axial positioningdevice 708. The first conical plug 702 and the second conical plug 704may be at different tapered angles relative to the test cell wall 706.The axial positioning device 708 may be utilized to move the firstconical plug 702 and the second conical plug 704 together orindependently. A brush 710 may be used to clean the test cell asdescribed above with respect to FIG. 3. This embodiment allows testingto be performed in either direction providing for investigation ofdifferent simulated fracture angles.

FIG. 8 depicts a test cell in accordance with yet another exemplaryembodiment of the present invention. A first conical plug 802 and asecond conical plug 804 are coupled to an axial positioning device 808and positioned so as to form different gap widths with the test cellwall 806. The axial positioning device 808 may be utilized to move thefirst conical plug 802 and the second conical plug 804 together orindependently. As would be appreciated by those of ordinary skill in theart, with the benefit of this disclosure, the simplicity of thisconfiguration allows for one pump rate to yield two shear rates.Consequently, rheology results may be obtained using a fixed geometry. Abrush (not shown) may be used to clean the test cell 800 as describedabove with respect to FIG. 3. This arrangement provides for a simplifiedtesting of rheology. In this embodiment, two different differentialpressures 810, 812 may be measured at one pump rate and the pump ratemay be fixed to approximate the required shear rate. In one exemplaryembodiment the shear thinning effect may be determined by comparing thedifferential pressure ratios measured at various pump rates.

In another exemplary embodiment (not shown), a third conical plug may beadded to the configuration illustrated in FIG. 8. The third conical plugprovides a third gap width, so that with three different constant pumprates the operating range of the measurements is increased to ninedifferent shear rates. The capability to manipulate the gap widths andthe flow rates makes it possible to selectively evaluate rheologicalbehavior in a low shear rate regime where viscoplastics exhibit their“yield stress” behavior and a broad class of shear thinning fluidsexhibit “Newtonian-like” behavior. Additionally, as would be appreciatedby those of ordinary skill in the art, with the benefit of thisdisclosure, rheological behavior may be evaluated in intermediate shearrate regime where details of the shear rate dependent viscosity functionare revealed, and in the “upper Newtonian-like” regime. Moreover, aswould be appreciated by those of ordinary skill in the art, with thebenefit of this disclosure, a broad range of shear rates enhances thecharacterization of flow behavior and the probability of defining therheological model that best describes the rheology of any fluid thatwill not plug the gaps.

This exemplary embodiment enhances the evaluation of the yield stressparameter which represents the minimum shear stress required to initiatea shearing flow and reflects the transition between solid-like (elastic,Hookean, etc.) behavior and viscous-like (Newtonian, shear thinning,etc.) behavior. This parameter is important in defining the flowbehavior of a class of systems that exhibit viscoplastic behavior, suchas certain formulations of drilling muds. As would be appreciated bythose of ordinary skill in the art, with the benefit of this disclosure,an increase in yield stress is followed by increases in apparentviscosities and annular pressure losses. It is also well known that anincrease in annular pressure loss is followed by an increase in theEquivalent Circulating Density (“ECD”). Hence the yield stress isparticularly important in minimizing excursions or upsets in “ECD”.

As would be appreciated by those of ordinary skill in the art, the ECDrepresents the effective hydraulic pressure exerted on the bottom of thewellbore by the combined effects of mud density and the total annularpressure loss resulting from hydraulic friction losses generated as thedrilling fluid circulates through the annular channels in the drillstring. As would be appreciated by those of ordinary skill in the art,with the benefit of this disclosure, it is desirable to maintain laminarflow in the annular channels of the drill string. Moreover, the annularfriction losses are highly sensitive to the value of the yield stress,with a lower yield stress indicating a lower total annular pressureloss. Consequently, a lower yield stress will reduce the contribution ofthe annular pressure loss to the ECD value. Moreover, as would beappreciated by those of ordinary skill in the art, with the benefit ofthis disclosure, although the present invention is described as using aconical plug, it is possible to use a plug having a different shape inanother embodiment without departing from the scope of the presentinvention. For instance, the plug may comprise a series of wedge shapedplugs and corresponding test cell walls.

Although the present invention is discussed herein in the context ofdrilling fluids, as would be appreciated by those of ordinary skill inthe art, with the benefit of this disclosure, the methods and systems ofthe present invention may be utilized in analyzing other fluids.Moreover, as would be understood by those of ordinary skill in the art,with the benefit of this disclosure, the characterization system 100 mayperform in one or any combination of the modes of operation discussedabove. For instance, in one exemplary embodiment the test cell 106 maybe utilized in a combined mode of operation thereby providing rheologymeasurement of the treated fluid as well as fluid optimization.

Therefore, the present invention is well adapted to attain the ends andadvantages mentioned as well as those that are inherent therein. Theparticular embodiments disclosed above are illustrative only, as thepresent invention may be modified and practiced in different butequivalent manners apparent to those skilled in the art having thebenefit of the teachings herein. Furthermore, no limitations areintended to the details of construction or design herein shown, otherthan as described in the claims below. It is therefore evident that theparticular illustrative embodiments disclosed above may be altered ormodified and all such variations are considered within the scope andspirit of the present invention. In addition, the terms in the claimshave their plain, ordinary meaning unless otherwise explicitly andclearly defined by the patentee.

1.-11. (canceled)
 12. A method of measuring the rheology of a firstfluid comprising: passing the first fluid through a gap formed between aconical plug and a conical portion of a test cell; measuring a pressuredrop along the gap; using the pressure drop measurement to determine ashear stress; measuring the flow rate of the first fluid through thegap; using the flow rate measurement and flow geometry to determine anaverage shear rate; and predicting rheological model parameters of thefirst fluid using the shear stress and the average shear rate.
 13. Themethod of claim 12, wherein the rheological model parameters areselected from the group consisting of: plastic viscosity, Bingham modelyield point and Herschel-Bulkley model parameters.
 14. The method ofclaim 12, further comprising: passing a second fluid through a gapformed between a conical plug and a conical portion of a test cell;measuring a pressure drop along the gap; using the pressure dropmeasurement to determine a shear stress; measuring the flow rate of thesecond fluid through the gap; using the flow rate measurement todetermine an average shear rate; predicting a rheological model of thesecond fluid using the shear stress and the average shear rate; andcomparing the rheological model of the first fluid and the second fluid.15. The method of claim 14, wherein the first fluid is a LCM laden fluidand the second fluid is a LCM particle free fluid. 16.-21. (canceled)